J . Am. Chem. SOC.1986, 108, 3515-3516
which on X-ray analysis fully confirmed the preceding and subsequent stereochemical assignments." Hydrogenation of the two enone diester E isomer mixture (6c,d) followed by C-3 epimerization, methylenation, and i-Bu2A1H as described also gave ca. 10% of pure diol 10. The somewhat unstable dialdehyde 12 in DME was added by syringe pump over 24 h to a refluxing suspension of McMurry Ti reagent from Zn-Cu and TiC13 in DME.14 After a further 18 h a t reflux, neutral workup and chromatography over Si gel/AgN03 using 15:l hexane-ether gave the single taxane triene 1315in 20% yield, accompanied by ca. 10% of a CzoH3202 diene diol established by NMR and by X-ray analysis of its crystalline (enol) monoacetate as the stable enol 15,arising from vinylogous reductive coupling of dialdehyde 1216 (Scheme I). The convergent phase of our synthesis leads from acetal 3 in 10 steps and 5% yield to the key dialdehyde 12,from which the sterically encumbered eight-membered B-ring can uniquely be formed by McMurry cyclization. To our knowledge this is the first direct cyclization of the taxane B-ring from any bicyclic seco-B intermediate. Moreover, triene 13 is not only the first synthetic compound containing the stereochemically correct taxane structure but offers attractive potential for taxusin synthesis. Thus 13 underwent selective allylic oxidation with Cr03/2,5-dimethylpyrazolel' to give enone 1418in ca. 44% yield. Enone 14 with MCPBA (5 equiv, CH2C12,room temperature, 1 h) undergoes smooth epoxidation at the C-4 methylene group, suggesting fruitful possibilities for selective B- and C-ring f~nctionalizations.'~ (14) (a) McMurry, J. E.; Kees, K. L. J . Org. Chem. 1977,42,2655. (b) Review: McMurry, J. E. Acct. Chem. Res. 1983, 16, 405. (15) 13: 400-MHz, 'H NMR, (CDC13,partial) d 5.90 (1 H, br d, J = 11.6 Hz). 5.15 (1 H. d. J = 11.6 Hz), 4.86 (1 H, s), 4.64 (1 H, s), 1.72 (3 H, s), I .07, 1.03,'and 0.86 (each 3 H, s). (16) Evidence for structure 15 will be detailed in our full paper. (17) Salmond, W. G.; Barta, M. A.; Havens, J. L. J . Org. Chem. 1978, 43, 2057. (18) 14: 400-MHz, 'H NMR (CDC13,partial) d 6.01 (1 H, br d, 12.5), 5.27 (1 H, d, 12.5), 4.86 (1 H, s), 4.64 (1 H, s), 2.90 (1 H, d d , J = 19.5,6.5 Hz), 2.10 (1 H, d, J = 19.5 Hz), 1.82 (3 H, s), 1.19, 1.14, and 0.92 (each 3 H, s). The 8 and J values for the C-14 a- and &protons at 2.9 and 2.1
parallel those. given for a related enone system by: Woods, M. C.; Nahnishi, K.; Bhacca, N. S. Tetrahedron 1966, 22, 243. (19) Partial support of this research by grant CA-18846, awarded by the National Cancer Institute, USPHS, DHHS, is gratefully acknowledged.
Ab Initio Predictions and Experimental Confirmation of Large Tunneling Contributions to Rate Constants and Kinetic Isotope Effects for Hydrogen Atom Transfer Reactions Bruce C. Garrett Chemical Dynamic Corporation Columbus, Ohio 43220
Donald G. Truhlar* Department of Chemistry, University of Minnesota Minneapolis, Minnesota 55455 Joel M. Bowman Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 Albert F. Wagner Chemistry Division, Argonne National Laboratory Argonne, Illinois 60439 Daniel Robie, Sivaram Arepalli, Nathan Presser, and Robert J. Gordon Department of Chemistry University of Illinois at Chicago Chicago, Illinois 60680 Received November 8, I985 The interpretation of kinetic isotope effects (KIEs) involves potential energy barrier heights, vibrational effects of stretches 0002-7863/86/ 1508-3515$01.50/0
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and bends, and competition between overbarrier and tunneling mechanisms.l In favorable cases, KIEs provide some of the most compelling evidence for or against detailed interpretations of the dynamics of reactive events. The present paper reports such a case in which the detailed question is the role of tunneling2 in hydrogen atom transfer reactions in the gas phase. Since many features of H transfer are similar to proton and hydride t r a n ~ f e r , ~ the role of tunneling in this kind of reaction has far reaching implications for reactions in solution as well as in gas-phase applications, such as combustion and atmospheric chemistry. In this communication we report new calculations and experiments on the bimolecular rate constant ratio k3/k4
o + HD -L, OH + D o + HD 2 OD + H which, together with earlier r e s ~ l t s for ~ - ~kl and k2
o + H, A OH + H 0 + D2 2 O D + D provide strong evidence for the dominance of tunneling in all four reactions at temperatures below 500 K. The KIEs were measured in two complementary experiments. In the first study6 kl, k2, and k3 k4 were measured with a flash photolysis apparatus,1° using atomic resonance fluorescence to monitor the decay of O(3P) in real time. In the new experiment the branching ratio k3/k4 was measured with a discharge flow apparatus using laser-induced fluorescence to determine the ratio of the steady-state concentrations of OH and O D products. Oxygen atoms were generated in a microwave discharge of N, containing 0.01% O2and combined far downstream with a mixture of either H D and N2 or H2, D2, and N,. The OH and OD fluorescence intensities observed with the H2/D2mixtures were used to normalize the fluorescence ratio obtained with HD.
+
(1) See, e.g., the following and references therein: (a) Melander, L.; Saunders, W. H., Jr. Reaction Rates of Isotopic Molecules, 2nd ed.; Wiley: New York,1980. (b) Garrett, B. C.; Truhlar, D. G. J. Am. Chem. Soc. 1980, 102, 2559. (c) Garrett, B. C.; Truhlar, D. G.; Wagner, A. F.; Dunning, T. H., Jr. J . Chem. Phys. 1983, 78, 4400. (d) Schatz, G. C.; Wagner, A. F.; Dunning, T. H., Jr. J. Chem. Phys. 1984.88, 221. ( e ) Tucker, S.C.; Truhlar, D. G.; Garrett, B. C.; Isaacson, A. D. J . Chem. Phys. 1985, 82, 4102. (2) The dominance of tunneling in H transfer is implied by comparing estimates of the rate constant with classical reaction coordinate motion to those with tunneling; see, e.g.: reference 1c.e. (a) Truhlar, D. G.; Kuppermann, A. J . Chem. Phys. 1972, 56, 2232. (b) Truhlar, D. G.; Kuppermann, A,; Adam, J. T. J . Chem. Phys. 1973,59,395. (c) Schatz, G. C.; Kuppermann, A. J . Chem. Phys. 1976, 65, 4668. (d) Truhlar, D. G.; Kuppermann, A,; Dwyer, J. Mol. Phys. 1977,33,683. (e) Truhlar, D. G. J . Phys. Chem. 1979, 83,188. ( f ) Garrett, B. C.; Truhlar, D. G. Proc. Natl. Acud. Sci. U.S.A. 1979, 76,4755; J . Chem. Phys. 1980, 72, 3460. (9) Garrett, B. C.; Truhlar, D. G . ; Grev, R. S.; Magnuson, A. W. J. Phys. Chem. 1980,84, 1730. (h) Blais, N. C.; Truhlar, D. G.; Garrett, B. C. J. Phys. Chem. 1981,85, 1094; J . Chem. Phys. 1982, 76, 2768. (i) Truhlar, D. G.; Isaacson, A. D.; Skodje, R. T.; Garrett, B. C. J . Phys. Chem. 1982.86, 2232. (j)Isaacson, A. D.; Truhlar, D. G. J . Chem. Phys. 1982, 76, 1380. (k) Lee, K. T.; Bowman, J. M.; Wagner, A. F.; Schatz, G. C. J . Chem. Phys. 1982, 76, 3583. (1) Skodje, R. T.; Truhlar, D. G.; Garrett, B. C. J . Chem. Phys. 1982,77,5955. (m) Clary, D. C.; Garrett, B. C.; Truhlar, D. G. J. Chem. Phys. 1983, 78, 777. (n) Truhlar, D. G.; Grev. R. S.;Garrett, B. C. J . Phys. Chem. 1983,87, 3415. ( 0 ) Garrett, B. C.; Truhlar, D. G. J . Chem. Phys. 1983, 79, 4931, 1984, 81, 309. (p) Truhlar, D. G.; Runge, K.; Garrett, B. C. In Twentieth Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1984; p 585. (3) Kreevoy, M. M.; Truhlar, D. G. In Iwestigation of Rates and Mechanism of Reacfion,4th ed.; BernasMni, C. F., Ed.; Wiley: New York, 1986; Part 1, p 13. (4) Schott, G. L.; Getzinger, R. W.; Seitz, W. A. Int. J . Chem. Kinet. 1974, 6, 921. (5) Westenberg, A. A.; deHaas, N. J. Chem. Phys. 1967,47,4241; 1969, 50. 2512. ~. ~I
(6) Presser, N.; Gordon, R. J. J . Chem. Phys. 1985,82, 1291. (7) Bowman, J. M.; Wagner, A. F.;Walch, S. P.; and Dunning, T. H., Jr. J . Chem. Phys. 1984,81, 1739. ( 8 ) Garrett, B. C.; Truhlar, D. G. Int. J . Quantum Chem., in press. (9) Sutherland, J. W.; Michael, J. V.;Nesbitt, F. L.; Klemm, R. B.; Pirraglia, A. N. In huenty-first Symposium International on Combustion,
Combustion Institute: Pittsburgh, accepted for publication. (10) Miller, J. C.; Gordon, R. J. J . Chem. Phys. 1983, 78, 3713.
0 1986 American Chemical Society
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J . Am. Chem. SOC.1986, 108, 3516-3517
Table I. Arrhenius Parameters for Reactions 1-4' i i T.K ICVT ICVT/ LAG CEOB/G experimentb 1 1400-1900 36, 14.0 35, 13.6 48, 14.4 37, 14c 3 18-471 6.0,10.7 0.6,7.9 0.7,7.95 0.4 f 0.1,7.8 f O.ld 1 2 422-472 1.4,0.8 0.7,1.8 0.4,2.3 0.3 f 0.2,2.4 f 0.5 1 3+4 422-472 1.3, 0.2 1.3, 0.2 0.9,0.5 2.3 f 1.1, -0.3 f 0.4 3 4 373-500 1.1, 0.5 0.4,1.9 0.3,2.1 0.5 f 0.1, 1.5 f 0.2 OLeft entry, A, in lo-'' cm3 molecule-1 s-I or unitless A,,; right entry, E, or E,, in kcal/mol. buncertainties are one standard deviation. eThe results of ref 4 over the T range indicated and of a fit over the same T range to the rate constants of ref 9 agree to the two significant figures shown. dReference 6. ~~
Combining the new measurements with previous data5s6on reactions 1 and 2 yields rate constants and their ratios for all four reactions. Further details will be published elsewhere." Three theoretical approaches are employed: improved canonical variational theory with classical reaction coordinate motion (ICVT),2g ICVT with a least-action ground-state transmission coefficient to account for tunneling (ICVT/LAG),& and reduced-dimensionality quantum calculations with an adiabatic incorporation of bending motion (denoted more specifically as collinear exact quantum with ground-state bend or CEQB/G).7 Details of the calculations for k3 and k4 are identical with those for kl and k2 reported el~where.'.~ The ICVT/LAG and CEQB/G calculations are our best theoretical estimates of the three-dimensional rate constants, whereas the ICVT calculations provide our best estimate of the classical reaction coordinate overbarrier contribution. Rate constants correspond to a sum of contributions from the 3Atand potential energy surfaces and are based on analytic fits7s8of ab initio calculations7Jz of these surfaces with no semiempirical parameters. The classical barrier height for both surfaces is 12.58 kcal/mol. For the present report previous results for reactions 1 and 2 and new results for 3 and 4 are fit to Arrhenius forms ki = Ai exp(-Ei/R7') and ki/kj = A, exp(E,JRT). Since the parameters depend on the temperature range used for the fit, this is indicated in every case. Tabel I first compares the experimental and theoretical Arrhenius parameters for kl at high and low temperatures. At 1400-1900 K, theory and experiment4s9agree well, whether or not tunneling is included. At 318471 K the preexponential factor and the activation energy are lower and the inclusion of tunneling lowers them further. The lower values are in much better agreement with experiment than are the ICVT values calculated without tunneling. We turn to the KIEs for a further confirmation of theory. In two of the three examples (kl/kz and k3/k4) the difference in activation energies and the ratio of preexponential factors seen in the experimental data can be explained only if tunneling is included. In the third case the predicted correction due to tunneling is too small to be discerned experimentally. In general there is good agreement between the ICVT/LAG and CEQB/G calculations and the experimental results. This provides confidence in the LAG method which can be used to estimate for each reaction the percentage of ground-state reactive events that occur by tunneling. At 400 K the estimated percentages are 76%, 57%, 80%, and 58% for reactions 1-4, respectively. The trends in Table I also indicate that the ICVT/ LAG method actually somewhat underestimates the tunneling effect so these are approximate lower bounds. We conclude that tunneling dominates these hydrogen-atom transfers at 400 K, and it must be included in interpreting the KIEs or in understanding or modeling the detailed dynamics. The LAG method predicts further that the percentage of reaction occurring by tunneling increases as the temperature is lowered, e.g., for reaction 1, to (11) Robie, D.;Arepalli, S.;Presser, N.; Williams, J.; Gordon, R. J., unpublished results. (12)(a) Walch, S.P.; Dunning, T. H., Jr.; Bobrowin, F. W.; Raffenetti, R. J. Chem. Phys. 1980,72,406.(b) Walch, S.P.; Wagner, A. F.; Dunning, T. H., Jr.; Schatz, G. C. J. Chem. Phys. 1980,72,2894.(c) Schatz. G. C.; Wagner, A. F.; Walch, S.P.; Bowman, J. M. J. Chem. Phys. 1981,74,4984. (d) Dunning, T. H., Jr.; Walch. S.P.; Wagner, A. F. In Potenrfal Encrgy Surfaces and Dynamics Calculations; Truhlar, D. G., Ed.; Plenum: New York, 1981;p 329.
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86% at 350 K and 94% at 300 K.
Acknowledgment. The work at Chemical Dynamics Corporation was supported in part by the US.Army Research Office; the work at the other institutions was supported by the US. Department of Energy, Office of Basic Energy Sciences. Registry NO.HD,13983-20-5; 0,17778-80-2.
Efficient Preparations of 8- and 9-Membered Cyclic Ethers Larry E. Overman,* Todd A. Blumenkopf,la Armando Castaiieda,lb and Andrew S.Thompson1c Department of Chemistry, University of California Imine, California 9271 7 Received March 3, 1986
The preparation of medium rings remains one of the more difficult problems in organic synthesis since cyclization reactions are compromised both by entropic effects and developing transannular interactions? The isolation3of a variety of medium-ring cyclic ethers from marine sources, e.g., laurencin (l), has
1
heightened interest in developing efficient methods for preparing these heterocycles. Progress in this area has recently been recorded4 by, among others, the groups of T r ~ s t K, ~~ c~i e n s k i , ~ ~ Schreiber," and Nicolaou.4d In this paper we demonstrate for the first time that eight- and nine-membered cyclic ethers containing A4 unsaturation can be prepared in good yields by direct cyclization of simple unsaturated acetals. We report also that the alkyl side chains characteristic of many eight-membered ring oxacyclic marine natural productsZcan be stereoselectivity introduced in the cyclization step. The method is illustrated by treatment of the (methoxyethoxy)methyl ether5*6of 5-hexen-1-01 (2) with 2 equiv of SnC14 at -20 OC for 13 h to give, in 83% yield, a 2:l mixture of cis3,6,7,8-tetrahydro-2H-oxacin(3) and 4-chlorooxocane (4).' (1) (a) NIH NRSA Postdoctoral Fellow (GM 09444), 1984, 1986. (d) COSNET-SEP Mexico Graduate Fellow, 1982-1986. (c) NIH NRSA Postdoctoral Fellow (CA 07787), 1985-1986. (2)See: Illuminati, G.; Mandolini, L.Arc. Chem. Res. 1981,14,102 and references cited therein. (3) Faulkner, D. J. Nut. Prod. Rep. 1984,251. (4) (a) Trost, B. M.;Verhoeven, T. R. J. Am. Chem. Soc. 1980,102,4743. (b) Cockerill, G. S.;Kocienski, P. J. Chem. Soc., Perkin Trans I 1985,2093, 2101. (c) Schreiber, S.;Kelly, S. E. Tetrahedron Left. 1984, 1757. (d) Nicolaou, K.C. Organic Seminar, University of California, Irvine, Jan 30, 1986. (5) Corey, E. J.; Gras, S.-L.;Ulrich, P . Tetrahedron Lett. 1976, 809. (6)Cyclization reactions were conducted in CH2C12at a concentration 0.02-0.05 M and were quenched with aqueous base prior to workup.
0 1986 American Chemical Society
